Graphene-Sulfur Nanocomposites for Rechargeable Lithium-Sulfur Battery Electrodes

ABSTRACT

Rechargeable lithium-sulfur batteries having a cathode that includes a graphene-sulfur nanocomposite can exhibit improved characteristics. The graphene-sulfur nanocomposite can be characterized by graphene sheets with particles of sulfur adsorbed to the graphene sheets. The sulfur particles have an average diameter less than 50 nm.

PRIORITY

This invention claims priority from U.S. Provisional Patent ApplicationNo. 61/390,945 entitled Graphene-Sulfur Nanocomposites forLithium-Sulfur Batteries filed Oct. 7, 2010.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ContractDE-AC0576RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

High-performance batteries can serve as part of a solution to supply andstorage problems and environmental issues related to the replacement offossil-fuel-based energy with clean alternative energy. Lithium-sulfurbatteries, in particular, are of interest because of the hightheoretical specific energy density (2600 Wh kg⁻¹), high theoreticalspecific capacity (1680 mAh g⁻¹), low material cost, and low safetyrisk. However, the poor electrical conductivity of elemental sulfur, thedissolution and shuttling of polysufulfide intermediates, and theresultant poor cycling performance limits the applicability andusefulness of Li—S batteries. Accordingly, a need exists for Li—Sbatteries that exhibit improvements in reversible capacity, ratecapability, and cycling stability.

SUMMARY

The present invention includes a rechargeable lithium-sulfur batteryhaving a cathode characterized by a nanocomposite comprising graphenesheets with particles comprising sulfur adsorbed to the graphene sheets.The sulfur particles have an average diameter less than 50 nm. Theinvention further includes methods for making the nanocomposite graphenesheets. Batteries based on embodiments of the present invention can havea reversible capacity greater than 950 mAh g⁻¹ even after 100 cycles. Insome embodiments, the tap density of the graphene-sulfur nanocompositepowder is preferably greater than 0.92 g cm⁻³. Furthermore, the sulfurcontent in the nanocomposite is preferably greater than approximately 70wt %.

The graphene sheets can be arranged randomly, pseudo-randomly, or in alayered stack. In the random arrangement, graphene sheets and/or regionsof graphene sheets having adsorbed sulfur particles do not exhibit arecognizable pattern in the arrangement of graphene sheets. The layeredstack can comprise adsorbed particles arranged in sulfur layers betweengraphenc sheets and/or layers of graphene sheets, wherein the sulfurlayers anl graphene layers substantially alternate. The pseudo-randomarrangement can comprise a mixture of random and stacked phases ofgraphene sheets.

In a preferred embodiment, the cathode comprises a polymer contactingthe nanocomposite to minimize diffusion of polysulfide species into theelectrolyte. The polymer can be applied to coat the nanocompositesurfaces. Alternatively, the polymer, the graphene sheets, and thesulfur particles can compose a mixture. Preferably, the polymer is acationic membrane. A particular example, includes, but is not limited toa sulfonated tetrafluoroethylene based fluoropolymer-copolymer.Batteries having such a polymer can exhibit a discharge capacity of atleast 74% of an initial capacity even after 50 cycles at 0.1 C. Analternative example of a polymer includes, but is not limited to,polyethylene oxide (PEO).

According to one embodiment of the present invention, thegraphene-sulfur nanocomposite having graphene sheets with adsorbedsulfur particles can be prepared by first thermally expanding a graphiteoxide to yield graphene sheets and then mixing the graphene sheets witha first solution comprising sulfur and carbon disulfide. The carbondisulfide is evaporated to then yield a solid nanocomposite, which isground to yield the graphene-sulfur nanocomposite powder having primarysulfur particles with an average diameter less than approximately 50 nm.

The polymer described elsewhere herein, can be applied by mixing thegraphene-sulfur nanocomposite with a second solution comprising apolymer and a solvent and then removing the solvent, according to oneembodiment.

The purpose of the foregoing abstract is to enable the United StatesPatent and Trademark Office and the public generally, especially thescientists, engineers, and practitioners in the art who are not amiliarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The abstract is neither intended to define theinvention of the application, which is measured by the claims, nor is itintended to be limiting as to the scope of the invention in any way.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions, the various embodiments, includingthe preferred embodiments, have been shown and described. Includedherein is a description of the best mode contemplated for carrying outthe invention. As will be realized, the invention is capable ofmodification in various respects without departing from the invention.Accordingly, the drawings and description of the preferred embodimentsset forth hereafter are to be regarded as illustrative in nature, andnot as restrictive.

DESCRIPTION OF DRAWINGS

Embodiments of the invention are described below with reference to thefollowing accompanying drawings.

FIG. 1 is an illustration depicting a graphene-sulfur nanocompositearranged in an ordered stack according to one embodiment of the presentinvention.

FIGS. 2 a and 2 b are cross-section transmission electron microscope(TEM) images at two different magnifications of a graphene-sulfurnanocomposite arranged in a layered stack according to embodiments ofthe present invention.

FIGS. 3 a-3 d include graphs providing data on the electrochemicalproperties of graphene-sulfur nanocomposite cathodes synthesizedaccording to embodiments of the present invention.

FIG. 4 is a graph depicting the voltage versus specific capacity of agraphene-sulfur nanocomposite cathode having an applied polymeraccording to embodiments of the present invention.

DETAILED DESCRIPTION

The following description includes the preferred best mode of oneembodiment of the present invention. It will be clear from thisdescription of the invention that the invention is not limited to theseillustrated embodiments but that the invention also includes a varietyof modifications and embodiments thereto. Therefore the presentdescription should be seen as illustrative and not limiting. While theinvention is susceptible to various modifications and alternativeconstructions, it should be understood, that there is no intention tolimit the invention to the specific form disclosed, but, on thecontrary, the invention is to cover all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe invention as defined in the claims.

A graphene-sulfur nanocomposite comprising a stack of alternating layersof graphene sheets and sulfur particles was synthesized according toembodiments of the present invention. 80 mg of graphene sheets wasprepared by thermal expansion of graphite oxide and 3.2 g of a 10 wt %solution of sulfur in carbon disulfide (CS₂), which were mixed together.The mixture was sonicated for 10-15 min and evaporated in a hood toexclude CS₂ while stirring in nitrogen gas. The drying sample was heatedat 155° C. with protection of nitrogen gas to better load sulfur on thesurface of graphene. Once the CS₂ had been substantially removed,thereby forming a solid nanocomposite, the solid nanocomposite wasground by using high-energy ball milling for 8 h. After grinding, theamount of sulfur in the graphene-sulfur nanocomposite was determined tobe about 71.8 wt % by a thermogravimetric analyzer in argon at a scanrate of 10° C./min from room temperature to 800° C.

A polymer coated graphene-sulfur nanocomposite was also synthesized. 100mg of of a graphene sulfur nanocomposite formed according to embodimentsof the present invention was mixed with 0.5 g of a 0.1 wt % NAFION®(e.g., sulfonated tetrafluoroethylene based fluoropolymer-copolymer)solution. The mixture was stirred continuously overnight and then heatedto 80° C. under stirring to evaporate the solvent from the NAFION®solution. The NAFION®-coated graphene-sulfur nanocomposite was obtainedby drying under vacuum to remove any residual solvent.

For electrochemical characterization, graphene-sulfur nanocompositepowders, synthesized according to embodiments of the present invention,were used to prepare cathodes. 80 wt % graphene-sulfur nanocompositepowder, 10 wt. % SP-type carbon black, and 10 wt. % polyvinylidenedifluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP) werecombined to form a slurry. The electrode slurry was then cast onto Alfoil. Electrochemical tests of the electrode materials were performedusing coin cells with the graphene-sulfur nanocomposite cathode andlithium metal as both counter and reference electrode. The electrolytewas 1M Lithium Bis(trifluoromethane)sulfonamide lithium (LiTFSI)dissolved in a mixture of 1,3-dioxolane (DOL) and dimethyoxyethane (DME)(1:1 by volume). The separator used was a microporous membrane (CELGARD®2400) and the cells were assembled in an argon-filled glove box. Thegalvanostatic charge-discharge test was conducted at a voltage intervalof 1.0-3.0 V by a battery testing system. Cyclic voltammetricmeasurements were also carried out with the coin cell at a scan rate of0.1 mV s⁻¹ using an electrochemical interface.

FIGS. 1-4 show a variety of aspects, experimental results, andembodiments of the present invention. FIG. 1 is a schematic illustrationdepicting a graphene-sulfur nanocomposite arranged in an ordered stack.Graphene sheets 100 and layers of adsorbed sulfur particles 101alternate in the stacks. In an alternative arrangement (notillustrated), the graphene sheets with adsorbed sulfur particles can berandomly arranged.

FIG. 2 a is a cross-section TEM image of a graphene-sulfur nanocompositeshowing large domains of layered material. The high resolution TEM imagein FIG. 2 b shows the alternating layers of graphene (low contrast/lightregions) 201 and layers of adsorbed sulfur particles (high contrast/darkregions) 202. In this particular embodiment, the sulfur particles areless than or equal to approximately 20 nm in diameter.

The electrochemical properties of a graphene-sulfur nanocomposite basedon embodiments of the present invention were tested using a cyclicvoltammogram (CV) and a constant current charge-discharge measurement. ACV curve of is shown in FIG. 3 a. Since the graphene only plays a roleas an electronic conductor and does not contribute to the capacity inthe potential region, the CV characteristics shown in FIG. 3 a can onlybe attributed to the intrinsic reduction and oxidation of sulfur,showing two reduction peaks and one oxidation peak. According to theelectrochemical reduction mechanism of sulfur electrodes, the reductionpeak around 2.3V is related to the reduction of the elemental sulfurdissolved in the electrolyte to lithium polysulfide (Li₂S_(n), 4≦n<8)and the other reduction peak at 2.0V is attributable to a decrease ofpolysulfide chain length and eventual formation of Li₂S. During areversed anodic scan, only one oxidation peak appeared at 2.5V,suggesting that the peaks of the two oxidation reactions are too closeto distinguish. The large overpotential observed for the second redoxreaction implies a high polarization could occur when transforming fromlithium polysulfide to Li₂S. This is due to the fact that overcoming thechange of chain length requires higher activation energy. FIG. 3 b showsthe first charge-discharge profile of the graphene-sulfur nanocompositeat a constant current of 168 mA g⁻¹(conesponding to a 0.1C rate). Thedischarge curve showed a two-stage discharge profile, corresponding tothe two types of discharge reactions, in good agreement with the CVresults shown in FIG. 3 a. The graphene-sulfur nanocomposite electrodedelivered an initial discharge capacity of 967 mAh g⁻¹ but exhibited 52%fade after 50 cycles as shown in FIG. 3 c. This shows that the layerednanostructure with alternating graphene and sulfur layers provides ahighly conductive, active framework but migration of soluble polysulfidespecies during cycling must be reduced.

Accordingly, in preferred embodiments, a polymer is applied to thegraphene-sulfur nanocomposite to further control the soluble sulfurspecies. Scanning electron microscopy (SEM) images (not shown) ofNAFION®-coated and uncoated nanocomposites show that the polymer cancoat the particle surface of the graphene-sulfur nanocomposite toprohibit diffusion of the polysulfide.

Referring to the graph of capacity as a function of cycle number in FIG.3 c, the NAFION®-coated graphene-sulfur nanocomposite electrode retains79.4% of the initial capacity after 50 charge/discharge cycles,exhibiting good cycling stability. Additional stability and ratecapability performance of the NAFION®-coated graphene-sulfurnanocomposite electrode is shown in FIG. 3 d. Though the initialdischarge capacity changes very little before and after coating, theNAHON®-coated graphene-sulfur nanocomposite retains 74.3% of the initialcapacity after 100 cycles at 0.1 C. FIG. 4, shows the voltage profileversus specific capacity of the NAFION-coated graphene-sulfurnanocomposite at various discharge rates (1C=1680 mA g⁻¹). Thenanocomposite cathodes deliver 839, 647 and 505 mAh g⁻¹ at 0.2C, 0.5Cand 1C respectively, corresponding to 89%, 69% and 54% retention of thedischarge capacity measured at 0.1C. The improved rate capability andhigh cycling stability of the NAFION®-coated electrode can be attributedto the high electronic conductivity of the graphene layers and thereduced polysulfide dissolution/migration provided by the NAFION®coating. The applied polymer coating appears to provide improvedmechanical strength in addition to improved chemical and electrochemicalstability. In particular, a sulfonated tetrafluoroethylenefluoropolymer-copolymer can form dense films to coat the surface ofgraphene-sulfur nanocomposites, which inhibit the polysulfide fromdiffusing into the electrolyte from the adsorbed sulfur particles.Furthermore, since it is a cationic membrane with sulfonate ionicgroups, Li ions readily diffuse through the membrane, while stillsuppressing polysulfide anion transport, most likely due toelectrostatic repulsion.

While a number of embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims, therefore, areintended to cover all such changes and modifications as they fall withinthe true spirit and scope of the invention.

1. A rechargeable lithium-sulfur battery comprising a cathode and anelectrolyte, the cathode characterized by a nanocomposite comprisinggraphene sheets with particles comprising sulfur adsorbed to thegraphene sheets, the particles having an average diameter less thanapproximately 50 nm.
 2. The battery of claim 1, having a reversiblecapacity greater than 950 mAh g⁻¹ after 100 cycles.
 3. The battery ofclaim 1, further comprising a polymer contacting the nanocomposite tominimize diffusion of polysulfide into the electrolyte.
 4. The batteryof claim 3, wherein the polymer coats the nanocomposite surfaces.
 5. Thebattery of claim 3, wherein the polymer, the graphene sheets, and thesulfur particles compose a mixture.
 6. The battery of claim 3, whereinthe polymer is a cationic membrane.
 7. The battery of claim 3, whereinthe polymer comprises a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer.
 8. The battery of claim 1, having a dischargecapacity of at least 74% of an initial capacity after 50 cycles at 0.1C.
 9. The battery of claim 3, wherein the polymer comprises polyethyleneoxide (PEO).
 10. The battery of claim 1, wherein a powder of thenanocomposite has a tap density greater than 0.92 g cm⁻³.
 11. Thebattery of claim 1, having a sulfur content greater than approximately70 wt % in the nanocomposite.
 12. The battery of claim 1, wherein theadsorbed particles are arranged in sulfur layers between graphene layersin a stack of alternating graphene layers and sulfur layers.
 13. Arechargeable lithium-sulfur battery comprising a cathode and anelectrolyte and having a reversible capacity greater than 950 mAh g⁻¹after 100 cycles, the cathode characterized by a nanocompositecomprising graphene sheets with particles comprising sulfur adsorbed tothe graphene sheets, the particles having an average diameter less than50 nm, wherein the sulfur content is greater than approximately 70 wt %in the nanocomposite.
 14. A method of preparing a graphene-sulfurnanocomposite for a cathode in a rechargeable iithium-sulfur battery,the graphene-sulfur nanocomposite comprising graphene sheets withparticles comprising sulfur adsorbed to the graphene sheets, the methodcharacterized by the steps of: Thermally expanding graphite oxide toyield graphene sheets; Mixing the graphene sheets with a first solutioncomprising sulfur and carbon disulfide; Evaporating the carbon disulfideto yield a solid nanocomposite; and Grinding the solid nanocomposite toyield the graphene-sulfur nanocomposite having sulfur particles with anaverage diameter less than approximately 50 nm.
 15. The method of claim14, further comprising mixing the graphene-sulfur nanocomposite with asecond solution comprising a polymer and a solvent, and then removingthe solvent.
 16. The method of claim 15, wherein the polymer is acationic membrane.
 17. The method of claim 15, wherein the polymercomprises a sulfonated tetrafluoroethylene based fluoropolymercopolymer.18. The method of claim 15, wherein the polymer comprises PEO.
 19. Themethod of claim 14, wherein the battery has a discharge capacity of atleast 74% of an initial capacity after 50 cycles at 0.1 C.
 20. Themethod of claim 14, wherein a powder of the grapheme-sulfurnanocomposite has a tap density greater than 0.92 g cm⁻³.
 21. The methodof claim 14, further comprising forming a stack of alternating grapheneand sulfur layers, the sulfur layers comprising adsorbed particlesbetween graphene layers.
 22. The method of claim 14, wherein therechargeable lithium-sulfur battery has a reversible capacity greaterthan 950 mAh g⁻¹.
 23. The method of claim 14, wherein thegraphene-sulfur nanocomposite has a sulfur loading that is greater than70 wt %.